With the recent advancements in interplanetary coronal mass ejection (ICME) imaging it is necessary to understand how heliospheric images may be interpreted, particularly at large elongation angles. Of crucial importance is how the current methods used for coronal mass ejection measurement in coronagraph images must be changed to account for the large elongations involved in the heliosphere. In this review of theory we build up a picture of ICME appearance and evolution at large elongations in terms of how it would appear to an observer near 1 AU from the Sun. We begin by revisiting the basics of Thomson scattering describing how ICMEs are detected, in this we attempt to clarify a number of common misconceptions. We then build up from a single electron to an integrated line of sight, consider the ICME as a collection of lines of sight and describe how a map of ICME appearance may be developed based on its appearance relative to each line of sight. Finally, we discuss how the topology of the ICME affects its observed geometry and kinematic properties, particularly at large elongations. This review is the first of a three-part series of papers, where a review of theory is presented here and a model is developed and used in subsequent papers.

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[1] The Solar Mass Ejection Imager (SMEI) has been tracking coronal mass ejections (CMEs) from the Sun to the Earth and beyond since it came online in February 2003. This paper presents some results from the first 19 months of data from SMEI, when over 140 transients of many kinds were observed in SMEI's all-sky cameras. We focus specifically on 20 earthward directed transients, and compare distance-time plots obtained from the SMEI transients with those observed in halo CMEs by Large-Angle Spectrometric Coronograph (LASCO) aboard Solar and Heliospheric Observatory (SOHO), and the arrival time of the shock observed by ACE at 0.99 AU. The geometry of one particular transient is compared using both LASCO and SMEI images in a first attempt to investigate geometry evolution as the transient propagates through the interplanetary medium. For some events, the halo CME, SMEI transient, and shock at 0.99 AU do not match, suggesting that some transients may not correspond to a halo CME. Finally, an evaluation of the potential of SMEI to be used as a predictor of space weather is presented, by comparing the transients observed in SMEI with the 22 geomagnetic storms which occurred during this timeframe. A transient was observed in 14 cases, and distance-time profiles would have allowed a prediction of the arrival time at ACE within 2 hours of its actual arrival for three events, and within 10 hours for eight events. Of these eight events, seven were detected by SMEI more than 1 day before the transient's arrival at the Earth.

A CME which was first seen in LASCO is tracked through SMEI and on out to Ulysses. These measurements allow us to determine the deceleration and compare different models of the deceleration process. It is found that both a simple "snow plough" model and an aerodynamic drag model predict a much more rapid deceleration in the lower solar wind than is observed. Therefore some driving force is needed over an extended range of distances to account for the motion of the transient. It is conjectured that at least part of this driving force may be provided by one of two low-latitude coronal holes which were close to the site of the CME.

We present images of solar wind electron density structures at distances of 1 A.U., extracted from the STEREO/HI-2 data. Collecting the images requires separating the Thomson-scattered signal from the other background/foreground sources that are 10 3 times brighter. Using a combination of techniques, we are able to generate calibrated imaging data of the solar wind with sensitivity of a few ×10 −17 B ⊙ , compared to the background signal of a few ×10 −13 B ⊙ , using only the STEREO/HI-2 Level 1 data as input. These images reveal detailed spatial structure in CMEs and the solar wind at projected solar distances in excess of 1 AU, at the instrumental motion-blur resolution limit of 1 • -3 • . CME features visible in the newly reprocessed data from December 2008 include leading-edge pileup, interior voids, filamentary structure, and rear cusps. "Quiet" solar wind features include V shaped structures centered on the heliospheric current sheet, plasmoids, and "puffs" that correspond to the density fluctuations observed insitu. We compare many of these structures with in-situ features detected near 1 AU. The reprocessed data demonstrate that it is possible to perform detailed structural analyses of heliospheric features with visible light imagery, at distances from the Sun of at least 1 AU.